The invention concerns a light barrier with a light emitter spaced apart from a light receiver, the emitter including a light source and transmission optics and emitting a substantially parallel light beam. The light receiver and the light emitter are arranged to be aligned with each other for focusing the light beam from the light emitter by means of reception optics on a reception element for generating an electrical signal as a function of the light received.
Such light barriers are used for the contactless detection of objects. In particular, when these light barriers are used to prevent accidents at machine tools, such as bending or edging presses, they must be exactly aligned during installation so that they function reliably. The necessary exact alignment is predicated upon the fact, among others, that the sending and receiving angle must be kept as small as possible (less than 2.5°), to prevent reflections and resulting failures to recognize objects. Likewise, for light barriers with small sending and receiving angles, the influence from neighboring systems and the sensitivity to stray light are very limited or entirely prevented. Special importance attaches to the alignment of a light transmitter and a light receiver in situations where the reception element is composed of multiple individual photocells arranged in line or matrix form. In such a case, a faulty alignment would have the result that the information contained in the distribution of light in the beam cross-section cannot be fully evaluated.
Besides the exact alignment during the installation of the light barrier, the corresponding attachment components must have a high degree of mechanical stability.
To make such alignments possible, a variety of alignment aids are known. When the distance from the light emitter to the light receiver is large, the alignment can be done with a permanently installed or externally adaptable sighting telescope. It is also known how to make alignments with the help of the light barrier's signal. In this method, the orientation of the light emitter and the light receiver is changed until the reception signal reaches a maximum. For this, it is known to change the pulsing frequency of an indicator diode as a function of the magnitude of the reception signal, for example.
Drawbacks of these known methods for adjusting the light barrier are, for example, in the sighting telescope solution, the associated costs and the amount of space required, and in the solution using the reception signal, the time it takes to attain the desired orientation.